There are several approaches for controlling buck-boost regulators. These include: (1) a conventional approach with a fixed output voltage; (2) cascading independent buck and boost regulators that both allow 100% duty cycle top switch operation and program their output voltages to create a pass-through region; and (3) placing a parallel current path (hot swap, ideal diode controller, load switch, or relay etc.) around the buck-boost regulator and using a window comparator to toggle between the buck-boost regulator and the parallel current path. Each approach, however, can have disadvantages, as now described.
A conventional fixed output voltage buck-boost regulator, for example, can include specifications that apply to automotive and industrial electronics and that often have a range of acceptable input voltages (e.g. LV224:9V-16V, LV248: 36V-52V). These may lend themselves to window regulation, whereas conventional buck-boosts regulate a fixed output voltage. Buck-boost regulators may be needed to maintain output voltage regulation during fault conditions (e.g. surges, load dump, cold start), while the input and output voltages are close in value the vast majority of the time. Continuous switching of a buck-boost regulator can also degrade efficiency and generates EMI. This may be doubly true in the buck-boost region (VIN−VOUT) where all 4 power MOSFETs are switching. Methods to improve efficiency may offer improvement at light loads (discontinuous conduction mode, burst switching). Methods to improve EMI during switching (e.g., PCB layout, spread-spectrum switching frequency) may only partially solve the problem. The maximum current/power dissipation of a buck-boost regulator may also be limited by the maximum boost duty cycle when the input current and associated conduction losses reach a maximum.
Cascaded 100% duty cycle buck and boost regulators that create pass window may also suffer from problems. The inductor current may be uncontrolled in the pass-through window for the boost and buck regulators. Damage can result from excessive current in the inductor and power MOSFETs. Cascaded high-Q RLC tanks exist between input and output in the pass-through region and large amplitude ringing of inductor current and output voltage can result from line and load transients. This limitation may cause the pass-through window to be small in most applications. Startup may be poorly controlled when input is within pass-through region. Two inductors and two output capacitors may be required, degrading efficiency, and increasing solution size and cost. Four-switch buck-boost switching at pass window boundaries may not be possible.
Parallel path around buck-boost enabled with a window comparator may also suffer from problems. A parallel path switch device, if fully enhanced or linearly controlled, may be subject to SOA constraints and easily damaged. It may require a timer for protection, meaning an interruption in the output voltage may be possible. Quiescent current in the parallel path device may be high, if the ability to quickly enable and disable that path is retained. Transitioning between parallel path conduction and buck or boost regulation may cause large output sag due to startup delay/soft-start ramp as the inductor current may have to start from zero. The inductor may be helpful filtering input noise and taming extreme transients (e.g. circuit breaker events). It may be better to keep it in the current path. Boost capacitors used for the buck and boost top gate drivers may provide an ideal charge reservoirs to quickly turn on/off power MOSFETs. But the alternate path circuit may use a low value charging current from an internal charge pump and turn on may be slow.